This disclosure relates to compensation of thermally induced refractive index distortions in an optical gain medium or other optical element. Examples of thermally induced refractive index distortions may include lensing, higher order distortions, and/or other distortions.
Thermal lensing is a simple example of a thermally induced refractive index distortion. Thermal lensing is an effect induced by a spatially varying temperature in a medium through which light propagates such as, for example, a guiding medium configured to guide light, a gain medium configured for light amplification, and/or other media. The local change of the refractive index may be directly related to the local temperature rise or to stresses from non-uniform heating or the stress associated with expansion mismatches of mechanically connected components. Heating concurrent with the operation or use of such a medium may cause thermal lensing through one or more mechanisms. For example, when a beam of light propagates through the medium and the medium is cooled on its outer surface, the medium may be hotter or colder on the axis of the beam of light relative to outer regions of the beam causing a transverse profile of the refractive index of the medium.
FIG. 1 conceptually illustrates thermal lensing in medium 100 through which light propagates. Collimated beam 102 enters optically pumped medium 100 from the left. Absorption of power by the medium from the inputted “pump” beam may cause an outputted signal beam to be amplified. Such absorption may also cause heating of the medium and therefore create a temperature profile that causes a thermal lens to be experienced by the outputted signal beam. As absorbed heat dissipates away from beam axis 104, a temperature profile arises in medium 100. More specifically, medium 100 is hotter at beam axis 104 relative to outer regions of medium 100, as indicated by temperature profile 106. In temperature profile 106, T represents temperature and x represents the transverse position. Temperature profile 106 may be approximated as parabolic, however, medium 100 may exhibit other temperature profiles. This temperature profile induces thermal lensing in medium 100 by inducing a transverse profile of the refractive index of medium 100. Refractive index n may vary with temperature as dn/dT. As such, light propagating in medium 100 may be focused at focal point 108 and divergent beam 110 may be emitted from medium 100 to the right. Depending on the strength of the thermal lensing induced in medium 100, the physical dimensions of medium 100, and/or other factors, focal point 108 may reside within or outside of the medium 100.
In a different scenario than that shown in FIG. 1, a medium may be pumped by a laser. The signal beam itself can influence the temperature profile by the way it extracts energy from the pumped medium. However, the signal beam itself, without the pump beam, can cause heating that leads to a thermal lens. In general, both signal and pump may influence the shape and strength of a thermal lens.
Thermal-lens compensation of high-power, solid-state crystalline and glass lasers is an established part of solid-state laser engineering. Such compensation typically involves the use of a conventional negative spherical lens in series with the laser medium, with the focal length of the compensating lens being specified to compensate the thermal lens that develops at the desired operational conditions. In some cases, a monolithic structure may be employed, whereby the desired negative lens function is achieved by polishing the ends of the laser medium into concave surfaces with the radius of curvature of the concave surfaces being specified to provide the necessary compensation focal length.
Conventionally, fiber-based applications (e.g., lasers and amplifiers) have typically not displayed significant thermal lensing due to their inherent high operating efficiency and small transverse dimensions. However, as laser power is scaled to increasingly higher levels in fiber-based applications, and as the core diameter is increased to accommodate the higher power, a point is reached where thermal lensing must be compensated, for example, to avoid significant reductions in efficiency, beam quality, and/or other performance metrics. Traditional approaches for compensating thermal lensing in solid-state lasers are generally not applicable to fiber-based applications, because, in solid-state lasers, the thermal-lens focal lengths are typically much longer than the length of the laser medium. In high-power, fiber-based applications, however, the opposite may be true: the thermal focal length may be significantly shorter than the fiber length (e.g., by an order of magnitude or more). It is noteworthy that the distinction between solid-state and fiber-based lasers is becoming blurry. For example, stiff, rod-like fiber lasers are at the intersection of the traditional line of distinction.